1. Field of the Invention
The invention relates to a power control circuit, more particularly to a power control circuit for controlling rated power supplied to a load.
2. Description of the Related Art
High intensity discharge (HID) lamps have been widely used to replace incandescent lamps as a result of high light-emitting efficiency and long service life thereof. In actual use, since a high intensity discharge lamp has an equivalent impedance that increases with an activation period, in order to maintain a stable lumen output of the HID lamp, an electronic ballast is used to control rated power supplied to the HID lamp, thereby avoiding damage to the high intensity discharge lamp.
FIG. 1 illustrates a conventional electronic ballast for an HID lamp (L) that includes a filter and rectifying circuit 40, an active power factor correction circuit 10, a power control circuit 20, and a full-bridge rectify circuit 30.
The filter and rectifying circuit 40 filters and rectifies an AC voltage signal (vi), and outputs a DC voltage. The active power factor correction circuit 10 is a boost DC-to-DC converter consisting of a boost inductor (LBoost), a boost power switch (SBoost), a boost diode (DBoost) and a capacitor (Cdc1).
The active power factor correction circuit 10 boosts the DC voltage from the filter and rectifying circuit 40 to generate a boosted DC voltage, and supplies the boosted DC voltage to the capacitor (cdc1). The active power factor correction circuit 10 is operable so as to enable a current flowing through the boost inductor (LBoost) to track the DC voltage from the filter and rectifying circuit 40 by controlling the boost power switch (SBoost) in a known manner.
The power control circuit 20 includes: a buck DC-to-DC converter, which consists of a buck power switch (SBuck), a buck diode (DBuck), a buck inductor (LBuck) and a capacitor (cdc2), for outputting an output voltage that is converted into an AC voltage outputted to the HID lamp (L) by the full-bridge rectifying circuit 30; a voltage detecting unit (Kv) for detecting a voltage applied to the HID lamp (L) and for outputting a voltage signal in accordance with the voltage applied to the HID lamp (L); a current detecting unit (Ki) for detecting a current flowing through the HID lamp (L) and for outputting a current signal in accordance with the current flowing through the HID lamp (L); a multiplier for outputting a power signal in accordance with a product of the voltage signal and the current signal from the voltage and current detecting units (Kv, Ki); an error amplifier (EA) for amplifying a comparison result between the power signal and a first reference signal; a comparator (comp) for outputting a pulse width modulation signal in accordance with a comparison result between the amplified comparison result from the error amplifier (EA) and a second reference signal; and a driver receiving the pulse width modulation signal from the comparator (comp) and outputting a driving signal to a control input end of the buck power switch (SBuck) so as to control a duty cycle of the buck power switch (SBuck), thereby controlling rated power supplied to the HID lamp (L) through the full-bridge rectifying circuit 30.
In such a configuration, the power control circuit 20 consists of a large number of components, thereby resulting in relatively high costs. Particularly, the multiplier is an expensive component, and use of the same results in a relatively complicated layout and a larger size.